The present invention relates to improved methods for fluxless brazing. Particularly, the invention relates to techniques for conditioning or plating the surface of an aluminum substrate so as to improve its ability to receive a braze-promoting layer and uses for such conditioned substrates.
Various methods of bonding aluminum are known in the prior art. In the context of heat exchanger assemblies, which are characterized by thin aluminum components, bonding has heretofore commonly been effected in the prior art by furnace brazing, most commonly, by Controlled Atmosphere Brazing (CAB) flux and Vacuum Brazing (VB).
In Vacuum Brazing, the parts to be brazed are provided with sufficient quantities of magnesium, normally by providing Mg alloy constituents in the filler metal or in the aluminum components, such that, when brought to temperature in a brazing furnace under sufficient vacuum conditions, the magnesium becomes sufficiently volatile to disrupt the oxide layer present and permit the underlying aluminum filler metal to flow together. While this technique provides for good bonding, it is essentially a discontinuous process, resultant from the need to apply a vacuum, and thus, is relatively expensive. It is also difficult to control, as it is very sensitive to oxidizing conditions in the furnace atmosphere, and demands that onerous standards of material cleanliness be maintained. Further, the evaporation of the magnesium leads to condensation in the brazing furnace, which requires frequent removal, thereby further adding to costs.
In Controlled Atmosphere Brazing, the ability to braze does not result from mechanical disruption of the oxide but rather, from chemical modification of the oxide by a fluoride salt flux which is applied to the parts. As the name suggests, CAB brazing does not require that a vacuum be drawn, such that the process may readily be carried out on a continuous basis, most typically using an inert gas furnace. While this provides for some reduction in cost, this cost saving is partially offset by the necessity for integration of fluxing systems, many of which will suffer from variable flux loading. Moreover, after the flux has been applied, the flux can be susceptible to flaking, such that contamination of the article of manufacture can occur. The flux can also be difficult to apply, especially on internal joints; can cause problems in terms of furnace corrosion and cleanliness in the finished product. More importantly however, it has been found that the flux can lose activity when exposed to magnesium. Thus, this process is not suitable for brazing magnesium-enriched aluminum alloys. As magnesium is a commonly used alloying element in aluminum to improve, inter alia, strength, this reduces the attractiveness of CAB brazing.
An alternative method of bonding aluminum is described in U.S. Pat. No. 3,482,305. In this method, a bond-promoting metal of cobalt, iron, or, more preferably, nickel, is coated on a part to be brazed, in a manner more fully described in U.S. Pat. No. 4,028,200. If properly applied, the nickel reacts exothermically with the underlying aluminum-silicon (7-12%) alloy, thereby presumably disrupting the aluminum oxide layer, and permitting the underlying aluminum metal to flow together and join. Vacuum conditions are not required, such that this method overcomes the limitations of VB. Further, as this method does not require a fluroride flux, it is suitable for utilization with magnesium-enriched aluminum alloys, such as are beneficially utilized in heat exchanger construction, and thus, overcomes the drawbacks of CAB. As additional benefits, this process has utility in association with a wide variety of aluminum alloys. However, the bath described in U.S. Pat. No. 4,028,200 is taught to be useful only in the context of parts previously conditioned by caustic etching; provides for relatively slow plating; and has a relatively limited useful life, thereby resulting in significant cost.
Other mechanisms are known in the plating industry as being capable of providing the deposit of nickel upon aluminum. One very popular electroplating bath is the Watts bath, which is known to have some utility in plating decorative nickel on aluminum substrates, provided a surface pretreatment is first carried out. Preferably, a zincate layer is first applied, followed by a thin copper plate (eg. Rochelle-type copper cyanide strike solution) or a thin nickel plate (eg. neutral nickel strike, nickel glycolate strike), followed by the Watts bath. However, these preplate steps add cost, and in the case of copper, have deleterious environmental aspects, resultant from the use of cyanide. Copper has a further disadvantage in that it can negatively affect the corrosion resistance of aluminum products. Although it is possible to plate nickel directly on the zincate layer, the Watts bath is difficult to control in these circumstances, such that satisfactory adhesion or coverage of nickel is not always obtained. Further, addition of lead to the Watts bath reduces its plating rate, yet further limiting the attractiveness of the Watts bath, given the known benefits associated with the inclusion of lead in the nickel deposit.
According to one aspect, the invention comprises a method of manufacturing an article of manufacture for use in a fluxless brazing process, the method including the step of applying a braze-promoting layer including one or more metals selected from the group consisting of nickel, cobalt and iron, onto a bonding layer which includes one or more metals selected from the group consisting of zinc, tin, lead, bismuth, nickel, antimony and thallium and which is disposed on a substrate comprising aluminum, the junction of the bonding layer and substrate defining a target surface of the substrate.
According to another aspect, the invention comprises a method of manufacturing an article of manufacture for use in an improved fluxless brazing process, the method including the step of plating a braze-promoting layer including one or more metals selected from the group consisting of nickel and cobalt, onto a substrate including aluminum, the junction of the braze-promoting layer and the substrate defining a target surface of the substrate, wherein the application of the braze-promoting layer is preceded by mechanical abrasion of the substrate such that the target surface defines a plurality of reentrant edges.
According to a further aspect, the invention comprises a method of manufacturing an article of manufacture for use in a fluxless brazing process, the method including the step of electroplating a braze-promoting layer including one or more metals selected from the group consisting of nickel or cobalt, onto a substrate including aluminum, wherein the electroplating is carried out in an aqueous bath having a pH of from about 5 to 7 and including, in solution, said one or more metals.
According to a yet further aspect, the invention comprises a method of manufacturing an article of manufacture for use in a fluxless brazing process, the method including the step of plating a braze-promoting layer including nickel onto a substrate including aluminum, wherein the plating is carried out in an aqueous bath consisting of an aqueous solution of: from about 3 to about 20 weight percent of nickel sulfate; from about 3 to about 10 weight percent of nickel chloride; from about 6 to about 30 weight percent of a buffering salt selected from the group consisting of sodium citrate and sodium glyconate; from about 0.005 to about 1.0 weight percent of a lead salt selected from the group consisting of lead acetate and lead citrate; and ammonium, wherein the bath has a pH value in the range of about 3 to 12 and has a mole ratio of nickel:citrate:ammonium in solution of about 1:0.5-1.5:1-6.
According to yet another aspect, the invention comprises a method of manufacturing an article of manufacture for use in a fluxless brazing process, the method including the step of plating a braze-promoting layer including nickel onto a substrate including aluminum, wherein the electroplating is carried out in an aqueous bath consisting of an aqueous solution of nickel, citrate and ammonium, wherein the plating bath has a pH value in the range of about 5 to 12 and has a mole ratio of nickel:citrate:ammonium in solution of about 1:0.5-1.5:1-6.
According to still yet another aspect, the invention comprises an article of manufacture for use in an improved fluxless brazing process, including a substrate including aluminum; a bonding layer on the substrate which comprises one or more metals selected from the group consisting of zinc, tin, lead, bismuth, nickel, antimony and thallium; and a braze-promoting layer on the bonding layer including one or more metals selected from the group consisting of nickel, cobalt and iron.
Other advantages, features and characteristics of the present invention, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, the latter of which is briefly described hereinbelow.